javascript - AJAX Message Clear -

i finishing form user registration, , wanted know how clear messages generated dynamically ajax, when user clears input (backspace). example, let's enter valid input, clear backspaces - code shows valid input message, want switch empty string / no message when happens:

if ($namecheckcount < 1) {     if (preg_match("/^\w+@[a-za-z_]+?\.[a-za-z]{2,3}$/", $email)) {         echo 'this email available.';         exit();     } else {         echo 'you entered email invalid format.';         exit();     }    } else {     echo 'this email taken.';     exit(); } 

is there way can change above code have clear message if there no input after backspace?

secondly, once input validated (i have 1 main html file form, , 3 php files check username, password, , email validity through use of mysql, or in case of passwords, simple string match), can set sort of flag allow user submit? example, code checks email such:

function checkemail() {         "use strict";          var status = document.getelementbyid("email_status");         var email = document.getelementbyid("email").value;          if (!(email == "")) {             status.innerhtml = "checking...";             var request = new xmlhttprequest();             request.open("post", "email_check.php", true);              request.setrequestheader("content-type", "application/x-www-form-urlencoded");              request.onreadystatechange = function() {                 if (request.readystate == 4 && request.status == 200) {                     status.innerhtml = request.responsetext;                 }             }              var verify = "emailtocheck=" + email;             request.send(verify);         }     } 

this within html file form. once fields validated, there way allow user submit? 1 user may have unique combination of username , email fields stored in user table along other data, , no username can associated multiple usernames, , vice versa. thank in advance tips!